This invention concerns improvements relating to metal finishes and in particular to the electrodeposition of chromium to provide a decorative and protective surface finish.
More especially, the invention concerns the corrosion resistance of chromium finishes and how this can be improved at least for certain types of plating solutions giving benefits for the manufacture of articles having a decorative and protective chromium finish.
Electrodeposition of chromium from chromium plating baths is especially suited to provide consumer products with a tough, hard wearing surface finish that is corrosion resistant. As a result, industry has come to rely heavily upon chromium plating to provide an easily obtainable durable surface finish.
For many applications, particularly over zinc and brass, the normal process is to deposit copper, with bright nickel and chromium on top to produce a bright, hard wearing and corrosion resistant surface finish. In most copper alloy based situations the initial copper layer is omitted.
The chromium surface finish provides good corrosion resistance in many environments but, in the household kitchen or bathroom environment, chromium plated articles spend a significant proportion of their time covered in a film of water and soap and this has been found to lead to a reduced corrosion resistance.
The problem is believed to be due not to corrosion of the chromium deposit itself but the result of corrosion of the nickel layer where it is exposed due to faults in the chromium deposit either as formed or resulting from use.
The corrosion of the nickel is caused by electrical activity resulting from the interaction between the nickel and the chromium in the alkaline environment that exists in household kitchens and bathrooms due to the soaps and cleaners currently used. Also present in this environment from both the water and the cleaners, in varying amounts, is the chloride ion.
The corrosion reactions taking place in the alkaline environment produce soluble corrosion products allowing the corrosion to spread, eventually releasing the chromium from the corroding nickel and ultimately exposing the base metal. Corrosion resistance can be extended by the intermediate deposition of a layer of semi bright nickel beneath the bright nickel layer but this adds to manufacturing costs both in terms of increased production times and higher material costs and does not prevent the corrosion reactions from taking place once the nickel is exposed.
Chromium has two stable valency configurations, the hexavalent state and the trivalent state. Traditionally, metallic chromium deposits are obtained using plating solutions of hexavalent chromium. The reason for this is that the majority of decorative deposits obtained from plating solutions of hexavalent chromium are non porous continuous films which prevent the corrosion reactions taking place unless there is a fault in the chromium deposit either as formed or resulting from damage or wear in use so as to expose the underlying nickel.
In contrast, the deposits from the plating solutions of trivalent chromium are microporous films with holes through to the surface of the nickel underlayer. These holes are invisible to the naked eye but the nickel underlayer is exposed allowing the corrosion reactions to take place.
The known plating solutions of hexavalent chromium are typically based on the compound chromic oxide (CrO3). When dissolved in water, this forms chromic acid which is a strongly acidic, oxidising solution that has been found to be carcinogenic. Plating processes using these solutions therefore present a serious health and safety hazard to the people using them and a pollution risk to the environment.
The trivalent state is known to be comparatively benign and plating solutions of trivalent chromium are less aggressive. The use of plating solutions of trivalent chromium is therefore desirable to reduce the health and safety risks to the people using them and the pollution risk to the environment but to date has not been widely adopted because of increased chemical control required to maintain them at optimum operating conditions.
Thus, trivalent chromium plating solutions are more susceptible to the ingress of tramp metals and quickly require correction to maintain their efficiency. Hexavalent chromium plating solutions on the other hand are more robust and require very little routine attention.
Another problem is that existing plating solutions of trivalent and hexavalent chromium produce surface finishes of different colour. In particular, trivalent chromium plating is darker than hexavalent chromium plating.
This difference in colour is readily discernible when comparing parts plated with hexavalent chromium and parts plated with trivalent chromium. As a result of the colour mis-match, it is not possible to mix parts plated with trivalent chromium and parts plated with hexavalent chromium.
Thus, where several parts to be assembled or used together are chromium plated, care is required to ensure a colour match is achieved. This has resulted in the use of hexavalent chromium plating in preference to trivalent chromium plating because of the afore-mentioned problems associated with trivalent chromium plating solutions and finishes.
The present invention has been made from a consideration of the problems aforementioned.
To this end, it is an object of the present invention to provide a surface finish of chromium having improved corrosion resistance.
More particularly, it is a preferred object of the present invention to provide a surface finish of chromium produced by an electrodeposition process from an aqueous plating bath containing hexavalent or trivalent chromium ions having improved corrosion resistance to surface finishes of chromium currently available.
It is a further desired object of the invention to provide a surface finish of chromium having improved corrosion resistance which can be produced using a plating solution of trivalent chromium ions.
It is yet another preferred object of the invention to provide a surface finish of trivalent chromium having improved corrosion resistance which substantially matches the colour of existing surface finishes of hexavalent chromium.